We must thank (or perhaps blame) James Joyce for this word ever appearing in modern writing, since he helped to revive it by using it as a recurring reference to guilt in Ulysses in 1922.
Inwit had gone out of the language around the middle of the fifteenth century and it would have remained a historical curiosity had not Joyce and a few other writers of his time found something in it that was worth the risk of puzzling his readers. It was formed in Middle English from in plus wit, the latter meaning the mind as the seat of consciousness and intelligence (we continue the idea when we talk about native wit or we describe somebody as having a quick wit). To have inwit meant that you had an inward sense of what was right and wrong — a conscience.
Modern examples — they’re rare enough for the word to be extremely unlikely to be in anybody’s active vocabulary — almost always echo the complete phrase that Joyce employed, agenbite of inwit. This dates from 1340. In that year, a Kentish Benedictine monk, Dan Michelis of Canterbury (often called Michael of Northgate) translated a devotional manual from French into English and titled it Ayenbite of Inwit. Ayenbite, or agenbite, is literally “again-bite”, a literal translation of the Latin word meaning “remorse”. This has as its root the verb mordere, to bite (the Romans felt that remorse was the emotion that returned to savage you). The title meant “remorse of conscience”.
A couple of relatively recent examples:
His fate suggests the agenbite of inwit came too late; flaunting the gods even once reaped a classical reward.
Nova, by Samuel R Delany, 1968. That’s as he wrote it; Delany has made the classic mistake of confusing flaunt with flout.
“Not at all,” I said, “I’m very happily married,” but even as I said it I felt the agenbite of inwit, as if I were telling a lie.
Blithe Tomato, by Mike Madison, 2006.
Inwit presents no pronunciation difficulty, but agenbite is another matter — none of the few dictionaries that include it say how it should be spoken, perhaps because they don’t know. It’s not, after all, a word very likely to be heard over the dinner table.